Structural Testing of the Kwajalein DCCB
Figure 1. DCCB located on the west side of Kwajalein Island.
Figure 2. Results of structural testing.
The headline in the November 8, 1988 issue of Stars and Stripes in the Pacific read “Guerrillas Rocket Kabul.” It was Election Day in the US, and George H. W. Bush with his running mate Dan Quayle was about to beat Michael Dukakis and Lloyd Bentsen for the White House. The Soviet Union had decided to delay its agreed withdrawal of 100,000 troops from Afghanistan because of increased attacks by Moslem [sic] rebels. Amid this backdrop, we had arrived on Kwajalein Island a few days earlier in preparation for structural testing on what was called the DCCB (Defense Central Control Building) seen here in Figure 1.
The Strategic Defense Initiative had been drafted some five years earlier as a replacement for the existing US doctrine regarding the Soviet’s nuclear arsenal of Mutual Assured Destruction, or MAD. Rather than continuing to build more offensive weapons – ballistic missiles, the initiative proposed the construction of a vast missile defense system capable of neutralizing the Soviet offensive nuclear threat altogether. Among the specifications for the Strategic Defense Initiative was a requirement for a new ground based radar capable of detecting much smaller items, much further away, and in greater numbers: the GBR-X, X- band radar. Testing for the new radar was to be conducted from the rooftop of the DCCB located near the western tip of the island.
The Kwajalein Atoll is home to the Ronald Reagan Ballistic Missile Defense Test Site formerly called the Kwajalein Missile Range. This is one of the facilities where new technology for identifying and tracking high speed objects in the atmosphere is tested. The GBR-X was one such new technology and it was to be mounted to the DCCB roof. The radar was quite massive and had certain mounting requirements including one for the natural frequency of its support foundation. The DCCB had been constructed several years earlier for a different project and was being repurposed for this new application; it is a reinforced concrete structure that measures approximately 200 feet square by 100 feet tall.
We adhered several accelerometers to the roof as well as to structural locations at different elevations within the building interior. We monitored the building movement in response to both forced and random vibration excitation. The random vibration resulted primarily from the building response to the direction and speed of the prevailing wind. For the forced vibration, we designed, fabricated, and installed a 5700 pound shaker system; it was hydraulically actuated and capable of delivering constant amplitude force excitation over a frequency range of 2 to 6 Hertz. The mass was supported on steel rollers, and the connecting actuator anchored to the center of the roof structure with an L shaped steel frame. The transmitted load, as well as all of the accelerometer responses were monitored and recorded with a digital data acquisition system. The results of the natural frequency testing in the north/south direction of the building are shown in Figure 2. Frequency response data was also taken in the east/west direction.